A polyvinyl chloride resin is a kind of general-purpose resin which is used widely in building materials, daily commodities, such as wallpapers, artificial leathers, woven fabrics, sheets, films and the like, and industrial goods.
A polyvinyl chloride resin may be obtained in the form of a microparticle by mixing vinyl chloride monomers, alone or in combination with copolymerizable co-monomers, with an emulsifying agent, a buffering agent, and a polymerization initiator, and subjecting the mixture to microsuspension polymerization, emulsion polymerization, seed polymerization, etc., to prepare a polyvinyl chloride resin latex, and drying the latex.
The polyvinyl chloride resin obtained in this way is transformed into plastisol by the addition of a plasticizer, a stabilizer, a filler and the like, and then can be used for various purposes. Particularly, a vinyl chloride copolymer resin including vinyl chloride acetate as copolymerizable monomers can be used in sealers for automobiles (under body coat, body sealer), tile carpets and adhesive layers for tarpaulin. One of the main quality aspects required for vinyl chloride copolymer resins is viscosity stability at high temperature (30˜40° C.). The less the change in viscosity over time, the better the viscosity stability.
Generally, in order to reduce the initial viscosity and change in viscosity with time of a microsuspension-polymerized polyvinyl chloride resin, a process of adding a nonionic emulsifying agent to a polyvinyl chloride resin latex has been used. However, though the nonionic emulsifying agent is somewhat effective at lowering an initial viscosity, the addition of the nonionic emulsifying agent has a limitation in lowering a change in viscosity over time.
Further, seeds are used in emulsion polymerization or microsuspension polymerization to adjust the viscosity of a polyvinyl chloride resin under the control of the particle size thereof. For example, Korean Patent Publication No. 2011-0139326 discloses a method of preparing a polyvinyl chloride resin, in which emulsion polymerization is performed with seeds of two sizes, followed by drying in the presence of a nonionic emulsifying agent. However, this method suffers from the disadvantage of the polymerization process being complicated due to the continuously feeding of monomers, an emulsifying agent and a polymerization initiator during polymerization, requiring a lot of time for the growth of the seeds into large particles, and being high in initial viscosity and change in viscosity with time, compared to a microsuspension polymerization process free of seeds.